There is an increasing demand for network based information services, and one of the greatest areas for growth is in mobile or wireless information services. Mobile information services are delivered over wireless communication networks such as, for example, digital cellular communication systems. Presently, voice communication accounts for most of the traffic carried by such systems, but the ability of digital systems to provide other types of data is providing opportunities for system operators to compete with land based networks in offering data services to mobile customers. However, there are a number of issues to be resolved with wireless mobile data networks.
In a mobile environment, it is much more likely that a communication session between a client, such as a mobile station, and a server will be interrupted. Interruptions occur for a variety of reasons, such as during hand off between serving cells, or during periods of poor signal quality. During hand-off there is often a brief period during which no information is exchanged. This type of interruption, because it is predictable and expected, has a minimal effect on communication. However, unexpected interruptions from poor signal quality, or dropped connections between the mobile station and a base station, are much more difficult to deal with. Examples of such an interruption would include when the mobile station passes through a tunnel, when the battery of the mobile station runs out of charge, server overloads, and so on.
Typically, when a connection is lost, the user would have to re-connect and engage in another session with server. This occurs in land line networks as well, such as when point to point protocol (PPP) computer dial in modem connections are dropped by the telephony carrier. When a dial in connection is dropped, the user must dial in again, usually receiving a new address, and establish a new communication session. Any transfer of data in progress when the session was interrupted is lost. The same is true of mobile network connections. If a connection between the mobile station and the fixed network equipment is lost, it is likely that the communication session is not recoverable.
The notion of session is defined in the Open System Interconnection (OSI) model as layer 5 of the seven defined layers. The loss of a session because of a problem in a lower layer is not a requirement by any means, and in fact there are proposed specifications implying that a session should be persistent, and at least tolerant of suspensions and redirections initiated by the client, server, or bearer network. The Wireless Session Protocol Specification of Version 30 (April, 1998) of the Wireless Application Protocol, the disclosure of which is hereby incorporated by reference, describes a session model that provides for suspend and resume operations.
A bearer network is defined as the network over which data is carried, including protocol as well as physical media. For example, a typical desktop computer is capable of communicating over a circuit data/PPP bearer to a server, as well as by a LAN such as ethernet. There is an increasing number of mobile communication devices made which are capable of using several bearer network such as packet data, circuit data, short message service (SMS), all as understood in the art.
The ability of a session to float or roam from one bearer network to another would greatly simplify the operations of higher layer protocols in mobile communication systems. This would be possible by using a suspended session, switching bearer networks, the resuming the session. If a session were likewise automatically suspended upon an interruption, then resumable upon reconnection, a great deal of time could be saved by mobile users experiencing such interruptions, and the session would appear seamless.
A couple complications arise, however, as to how to go about implementing a seamless communication session, survivable among bearers and across multiple connections. For example, in wide area packet networks, such as the internet, packets or messages may linger in the network. This is because such networks are operate according to a "store and forward" principle. If the session changes bearers, or if the address of the client changes upon reconnection, the envelope identifier that lingering messages carry will no longer be valid. Consequently, with any current session protocol, these packets will be discarded upon reaching their destination. By envelope identifier it is meant a peer address quadruplet consisting of the client address, client port number, server address, and server port number. The server uses the envelope identifier to determine to which session the packet belongs. Typically, as in the proposed Wireless Application Protocol, a session identifier is a 32 bit field, and is considered too long to include in each packet, thus only the envelope identifier is used, placing the burden of matching packets with sessions on the network equipment.
A second problem that occurs is the problem of resolving aliases due to dynamic address assignment, as in PPP connections. When a circuit data connection is interrupted, and later resumed, the client will typically get a new address assignment, changing the envelope identifier it uses in sending packets. Packets with the old envelope identifier will be lost, resulting in the user having to re-transmit requests. Therefore, there is a need for a means to resolve the issues of lingering messages and changes in envelope identifiers in a communication system.